RESEARCH I NEWS
Lambing by the Mother of
All Techniques
Cloning by Nuclear Transplantation
Dolly
is
the
world's
first
mammalian clone to have been
created from a non-reproductive
cell of an adult animal.
D Somashekar
Cloning was done in sheep by nuclear
transplantation. A differential cell was
made to develop into a complete organism
by an unique gene cloning technique. The
lamb born by this method, named Dolly,
is the world's first mammalian clone to
have been created from a fully differentiated adult cell.
'Dolly' - one of the most unique forms of
life on our planet has created joy among a
section of scientists and at the same time
created terror among religious heads and
politicians. Yes, I am referring to the
innocen t sheep Dolly, named after an
American country singer, Dolly Parton. She
is the world's first mammalian clone to
have been created from a fully differentiated
non-reproductive cell of an adult animal.
She is an exact genetic 'xerox copy' of her
mother, born without a biological father.
Dr. Ian Wilmut and his team, from Roslin
Laboratory of Scotland created her using
what can literally be called the 'mother of
all techniques' in mammalian cloning.
Let us see in some detail how this supercreature came into this world. Cloning is a
technique by which one can propagate
cloned cells/organisms. This technique is
very popular in plant tissue culture, where
one can generate a whole plant from a cell
taken from any part of the plant and
cultured on specially designed media.
Somehow, this was not possible with animal
cells, and it was thought that because animal
cells are highly specialized, development
in these cells is irreversible. For example,
an udder cell in culture can give rise to
only other udder cells, not to skin cells or
an organ. Dolly is an exception to this
limitation and with her creation one of the
central dogmas of developmental biology
Box 1. What is a Clone?
A clone refers to a population of cells or organisms which were derived asexually from a single
ancestor. Thus, they are genetically identical to each other, and to their common ancestor. A number
of organisms naturally reproduce in this way, but adult mammals are not able to clone themselves.
Identical twins, incidentally, can be considered, to be 'clones' produced as a result of a chance
splitting of a fertilized egg in the course of normal, sexual reproduction.
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-\;J
---+
Udder of sheep
~
--+
Udder cell
1<..----
<~
®
(i)
---+
Udder
Udder cells
Reprogramming
~J
t-
EXPECTED
DEVELOPMENT
/;4---r-;
~~~
DEYELOPMENT
ACHIEYED
Figure 1. A schematic illustration
of differentiation.
Dolly
has been reversed. An adult mammalian
cell can evidently be made to remember
and reprogramme its development from
the beginning, thus producing a complete
viable organism (Figure 1).
In order to create Dolly, Wilmut took a cell
from the udder of a si~ year old ewe and
the cell was made to forget its existing role
of developing into another udder cell by
starving the cell with gradual reduction of
nutrients (reprogramming). An unfertilized
egg was then taken from another ewe and
from it the nucleus containing the genetic
material (DNA) was removed. Then the
nucleus from the reprogrammed udder cell
was taken and transplanted into the
nucleus-free egg cell (Figure 2). The
transplanted cell was allowed to grow and
divide in a culture dish. After a week or so
Donor sheep
Fusion by
electric shock
Egg nucleus removed
Figure 2. Cloning technique adopted for the
creation of Dolly.
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Box 2. Differentiation and Totipotency
The millions of cells in a multicellular individual, although originated from a single fertilized egg,
are not the same. Some may be udder cells, others skin cells, still others nerve cells, etc. They look
different and perform different functions. The processes by which these cells become specialized for
their specific tasks are collectively known as cellular differentiation.
Earlier experiments on development and differentiation have shown that nuclei from early embryos
can be used to produce adult clones or a complete viable organism. But this was not possible with the
nuclei of cells of embryos beyond a certain stage. Hence, the nuclei from early embryos seemed to
retain their potency to develop and differentiate into all types of cells (skin, udder, nerve cells, etc.)
and so they were called totipotent cells. On the other hand adult differentiated cells in animals are
typically not capable of giving rise to a complete viable organism or return to the state of totipotency.
Today, however, Dolly is a living testimony to the fact that, given the right conditions, even a fully
differentiated adult mammalian cell can be made to give rise to an entire organism.
this cell formed an early embryonic stage
called blastocyst. This was implanted into
the uterus of a third ewe that acted as a
surrogate mother. After 5 months of
gestation, at 4 pm on July 5, 1996, Dolly
was born and almost immediately triggered
off all kinds of controversies.
Although the technique seems simple in
narration, it took Wilmut over 10 years and
over Rs 2.5 crores in research funds to
achieve the goal of cloning a whole mammal
from a fully differentiated non-reproductive
An adult mammalian cell can
evidently be made to remember
and reprogram its development
from the beginning, thus
producing a complete viable
organism.
cell. In nature, during reproduction a sperm
from the male parent and the egg of the
female parent fuse. After development and
differentiation, the fertilized egg eventually
gives rise to an offspring. Thus, the sexually
produced offspring inherits half of its
genetic material from each parent. Dolly,
on the other hand, is an identical clone of
the ewe from which the udder cell was
taken and does not have any genetic
resemblance to either the ewe from which
the egg was taken, or to the surrogate
mother who bore and delivered her. Thus,
Dolly truly has only a single parent.
Another interesting fact is that Dolly was
born by a process of asexual reproduction,
even though there was fusion of a nucleus
with an egg that lacked its own nucleus.
The complete software (DNA) that controls
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RESEARCH I NEWS
It took Wilmut over 10 years and
over Rs 2.5 crores in research
funds to achieve the goal of
cloning a whole mammal from a
fully
differentiated
nonreproductive cell.
the unfolding of development was provided
by a somatic cell (in this case from the
udder) rather than a reproductive cell.
Asexual reproduction is commonly seen in
lower forms of animal life like amoeba,
hydra etc., but not in mammals.
Despite the furore created by this
achievement, the success rate in this cloning
endeavour was very low. Out of 277 nuclei
that were transferred to enucleated eggs,
only 29 eggs grew into embryos. Out of
these, only 13 of the embryos could be
successfully implanted into surrogate
mothers, and, of these 13, just one ewe was
successful in giving birth to an offspringDolly. The key to enhancing the success
rate of this technique seems to lie in
developing methods to make donor nuclei
more acceptable to the cytoplasm of the
recipient egg or oocyte.
Although the technique was a success in
the case of a single Dolly, it remains to be
seen whether it can be extended to other
animals. As some researchers point out,
Dolly could also have been a result of
inadequate removal of genetic material
(DNA) from the recipient egg. Even Wilmut
adtnits that Dolly's genetic material might
have come from a stem cell, which had
retained in its memory the complete story
of embryonic development. Well, the next
thing haunting the mind is 'can we clone
humans' by using this technique? If
everything works well, as in the case of
Dolly, it is certainly possible. Cloning has
already been attempted in humans and
monkeys using cells of the embryo, but
not from an adult non-reproductive cell.
Unlike in other mammals, in sheep there is
enough time gap to remodel the
transplanted adult nucleus to an embryonic
state. It remains to be seen if a human adult
cell/nucleus can be reprogrammed to an
embryonic state and whether it can fuse
harmoniously with an empty egg to begin
the thread of life. If that becomes a reality,
we may someday be able to manufacture
clones of celebrities. Whether we would
want to do so or not, is of course, another
question altogether.
Suggested Reading
•
•
Collin Stewart. An udder way of making lambs.
Nature. 385. pp. 769-771, 1997.
I Wilmut, A E Schneike, J McWhir, A J Kind
and K H S Campbell. Viable offspring derived
from fetal and adult mammalian cells. Nature.
385. pp. 810-813, 1997.
D Somashekar is a freelance writer. At present, he is
working as Research Associate in the Food
Microbiology Department at CFTRI, Mysore.
D Somashekar, Freelance writer, H.No.C/5S,
'Vidhihasya', Petenarayan Temple Street, Srirangapatna 571 438, India.
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